1-Go to http://www.wunderground.com/weather/api and create a free account
2-After creating the account you'll receive a key - by email or you may login on their site to view it
3-RRCONFIG (plugins) and set: weatherundergroundkey=xxxxxxxxxxxxxxxx (where xxxxxxxxxxxxx is your key)
4-I highly suggest cleaning up your weatherpresetidxx= settings in rr.ini (since the IDs are different)
5-Fire it up and try it

Please note there are limits to the free account: 10 queries per minute, and 500 queries per day. IMPORTANT: Each Weather reading takes 2 queries!! (One to obtain City info and another to obtain the weather for the city) - so you can check 5 locations in 1 minute, then wait another minute and check another 5 (up to 250 readings in 1 day).

This is a bit slower than the weather.com (mostly because we have to parse a large amount of data), but it is free.
I recommend setting your city name settings like this:
USA: <CITY>, <STATE> such as: Chicago, IL
Rest of the world: <CITY>, <COUNTRY> such as: Toronto, Canada

Google's weather is very limited. Only 2 days past today, and it will not return other cities... meaning, if you search for Miami, it will ONLY return the weather for Miami, Florida. It will not show Miami, AZ, Miami New Mexico ect ect... That, and just a lot of information is not there to retrieve. It is very basic.